I am using Odoo 12.0 (Community Edition), the odoo is working perfectly yesterday and I have made no change into the server.

When I wanted to create new contact or new Sales the error showed up. But when I edit old contact or old Sales it works Ok. Suddenly today the error showed up. What should I do to fix this?

Error: 
Odoo Server Error

Traceback (most recent call last):
File "/odoo/odoo-server/odoo/http.py", line 654, in _handle_exception
return super(JsonRequest, self)._handle_exception(exception)
File "/odoo/odoo-server/odoo/http.py", line 312, in _handle_exception
raise pycompat.reraise(type(exception), exception, sys.exc_info()[2])
File "/odoo/odoo-server/odoo/tools/pycompat.py", line 87, in reraise
raise value
File "/odoo/odoo-server/odoo/http.py", line 696, in dispatch
result = self._call_function(**self.params)
File "/odoo/odoo-server/odoo/http.py", line 344, in _call_function
return checked_call(self.db, *args, **kwargs)
File "/odoo/odoo-server/odoo/service/model.py", line 97, in wrapper
return f(dbname, *args, **kwargs)
File "/odoo/odoo-server/odoo/http.py", line 337, in checked_call
result = self.endpoint(*a, **kw)
File "/odoo/odoo-server/odoo/http.py", line 939, in __call__
return self.method(*args, **kw)
File "/odoo/odoo-server/odoo/http.py", line 517, in response_wrap
response = f(*args, **kw)
File "/odoo/odoo-server/addons/web/controllers/main.py", line 962, in call_kw
return self._call_kw(model, method, args, kwargs)
File "/odoo/odoo-server/addons/web/controllers/main.py", line 954, in _call_kw
return call_kw(request.env[model], method, args, kwargs)
File "/odoo/odoo-server/odoo/api.py", line 745, in call_kw
return _call_kw_model(method, model, args, kwargs)
File "/odoo/odoo-server/odoo/api.py", line 718, in _call_kw_model
result = method(recs, *args, **kwargs)
File "/odoo/odoo-server/odoo/models.py", line 1132, in default_get
ir_defaults = self.env['ir.default'].get_model_defaults(self._name)
File "<decorator-gen-45>", line 2, in get_model_defaults
File "/odoo/odoo-server/odoo/tools/cache.py", line 93, in lookup
value = d[key] = self.method(*args, **kwargs)
File "/odoo/odoo-server/odoo/addons/base/models/ir_default.py", line 151, in get_model_defaults
result[row[0]] = json.loads(row[1])
File "/usr/lib/python3.6/json/__init__.py", line 354, in loads
return _default_decoder.decode(s)
File "/usr/lib/python3.6/json/decoder.py", line 339, in decode
obj, end = self.raw_decode(s, idx=_w(s, 0).end())
File "/usr/lib/python3.6/json/decoder.py", line 357, in raw_decode
raise JSONDecodeError("Expecting value", s, err.value) from None
json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)